Add tray icon colour change behaviour based on ProbeStatus

[bryandhill]

Updated framework to 4.8 Added functionality to change tray icon colour based on ProbeStatus Added configuration option to enable (default disabled) tray icon colour change Fixed bug whereby tray icon persisted after application was closed, until mouse over

Currently the status only turns red for ProbeStatus == ProbeStatus.Down

Please advise if there is anything else I can/should do for this pull request.

[bryandhill]

I realize now that upgrading to 4.8 might not be ideal. Let me know if you want me to take it back to 4.5